Optimization arrangement for direct electrical energy converters

ABSTRACT

An arrangement is provided for supplying electrical energy to a load from a direct electrical energy converter that optimizes converter power generation efficiency. The arrangement for optimizing converter power generation efficiency includes an impedance transformation circuit coupled between the energy converter and load for regulating current delivered by the energy converter so as to maximize power delivered to the load.

FIELD OF THE INVENTION

The present invention relates to vehicular power systems, and moreparticularly to an optimization arrangement for both primary and directenergy converters.

BACKGROUND OF THE INVENTION

In most DC electrical power systems for automotive, aerospace andstationary applications, the electrical power requirements have beenincreasing dramatically over the last several years. There is an ongoingtrend to move to a 42-volt power system which is now being deployed inthe automobile industry in order to meet the increased electricalparasitic loads. The increasing use of electrical systems in automobilesand aircraft is driven by the introduction of new functionality whichwill be provided by these systems, and an inherently higher level ofcontrol when engine-driven loads are replaced with electrically-poweredversions.

One arrangement for addressing this rise in electrical powerrequirements uses direct energy converters (DECs) to recover heat andwaste energy and augment the current power plants in vehicles. DECsprovide electrical power over an extremely broad range of voltages,nominally 1 mV to several volts DC, but are typically stacked up inseries to provide voltages in excess of 300 volts DC. The load currentstypically range from 1 milliamp to 300 amps DC, as the power demand inDC electrical systems can vary widely depending upon the mode ofoperation and upon parasitic subsystems which randomly come on line.

If as stated above, DECs are utilized to augment the engine orpower-plant, and as such, improve their overall efficiency, it isfurther desirable that the energy converter itself be optimized tooperate at high efficiencies. The proposed system is introduced in orderto provide a control scheme (hardware and software) necessary to achievethese higher efficiencies. In addition, the proposed system could alsobe used to optimize or maximize the lifetime and stability of the DECenergy source.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a system supplieselectrical energy to a load from a direct electrical energy converterusing an arrangement for optimizing converter power generationefficiency. The arrangement for optimizing converter power generationefficiency includes an impedance transformation circuit coupled betweenthe energy converter and load for regulating current delivered by theenergy converter so as to maximize power delivered to the load.

In accordance with another aspect of the invention, a method is providedfor optimizing power generation efficiency of a direct electrical energyconverter applying electrical current to a load. The optimization methodincludes monitoring output current and output voltage of the directelectrical energy converter and monitoring current through and voltageacross the load. Next, an impedance transformation circuit is placedbetween the direct electrical energy converter and the load. Then, theoptimization method involves adjusting the impedance of the impedancetransformation circuit as a function of monitored energy convertercurrent and voltage and load current and voltage so as to maximize powerdelivered to the load.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a prior art circuit diagram of a typical system involving adirect energy converter where a DC/DC converter is used to regulate theenergy to the load;

FIG. 2 is a circuit diagram of a prior art series stacked generator foruse with the source optimization system;

FIG. 3 is a graph of the power, voltage and efficiency of a typicalthermoelectric generator employed as a direct energy converter for asingle thermoelectric device where conventional means are used toregulate the energy to the load;

FIG. 3 a is a graph of the power, voltage and efficiency for tenthermoelectric devices in series, where conventional means are used toregulate the energy to the load;

FIG. 4 is a circuit diagram of the optimization system according to theprinciples of the present invention;

FIG. 5 is a detailed circuit diagram of the source power optimizationsystem as shown in FIG. 4; and

FIG. 6 is a flowchart of a method implemented by the controller of FIG.4 for maximizing the efficiency-power product of the optimization systemof FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a conventional arrangement 10 is shown. Theconventional arrangement 10 has a DEC 12 which creates a current I_(S)and a voltage V_(S). The DEC 12 is coupled to a DC/DC converter 14.DC/DC converter 14 is coupled to a load 16. Current I_(S) flows from theDEC 12, through the DC/DC converter 14 to the load 16 and back to theDEC 12. The DC/DC converter 14 is used in situations where the energy tothe load voltage V_(S) of the DEC 12 is different from the voltagerequired by the load 16. The DC/DC converter 14 typically has anefficiency in the range of 90-99%, for power levels less than 1 kilowattand 85-95% for power levels between 1 to 50 kilowatts. The idea behindthe conventional optimization arrangement 10 is to apply a constantterminal voltage across or constant current through the load 16.However, this solution only addresses regulation of the load 16 and doesnot consider the internal efficiency or operating current of the DEC 12.Furthermore, the arrangement 10 does not necessarily optimize the powerbeing drawn from the DEC 12 or delivered to the load 16 throughimpedance matching of the DEC internal resistance and the loadresistance (real part of the load impedance).

As shown in FIG. 2, most DECs 12 are comprised of a number of individualDEC circuits 18 in order to generate the appropriate system power andvoltage requirements (up to several hundred individual DEC circuits 18in some cases). These individual DECs circuits 18 are usually configuredin “stacks” of either series or parallel circuit configurations toachieve different voltage conditions depending upon the specifictechnology. The DC/DC converter 14 shown in FIG. 2 does not optimize theDEC 12 according to which stacked configuration of the DEC 12 works bestfor a given situation; series, parallel or some hybrid of the two.

FIG. 3 shows the problems with the conventional optimization system 10.The voltage-current characteristic 20 of a typical thermoelectricgenerator (TEG) are displayed. For this curve, the thermoelectricdevices are connected directly to a load and hence the load current isequal to the source current. The thermal efficiency 22 and the powerdelivered to the load 24 as a function of current are also plotted. Theload power 24 is calculated from the power (I²R_(L)) developed in theload resistance, (R_(L)). The thermal efficiency is defined by the power(I²R_(S)(T)) lost in the source internal resistance, (R_(S)(T)), theSeebeck power (kΔT) produced by a thermal gradient applied across thedevice, and the Peltier effect (αTI_(S)). It can be seen that thecurrent 28 at which maximum TEG efficiency is achieved is not equal tothe current 30 at which maximum power is delivered to the load 32.

Furthermore, when ten thermoelectric devices are connected in series, asshown in FIG. 3 a, the discrepancy becomes even more pronounced. This isshown by the difference between the current 28 at which maximum TEGefficiency is achieved and the current 30 at which maximum power isdelivered to the load 32 when compared to FIG. 3.

With reference now to FIG. 4, an optimization system 100 for anelectrical power conversion system is shown. The optimization system 100includes a DEC 102. The DEC 102 is coupled to a source poweroptimization system (SPOS) 104. The SPOS 104 is further coupled inparallel to an energy storage device 106. The energy storage 106 deviceis also coupled in parallel to a load regulator 108, in this example aDC/DC converter. The load regulator 108 is connected in parallel to aload 110.

The DEC 102 is a generator which may be any voltage or current sourcesuch as a thermoelectric or thermoionic device, electrochemical battery,solar cell or photovoltaic converter, thermophotovoltaic system, fuelcell, plasma power generator, ferroelectric device, piezoelectricdevice, electrohydrodynamic generator and the like, which produces avoltage, (V_(S)), and results in a source current (I_(S)). The DEC 102could also function as a current generator, (I_(S)), with a subsequentcompliance voltage, (V_(S)), such as is the case with a photovoltaicdevice. The current I_(S) flows from the DEC 102 to the SPOS 104.

In an exemplary embodiment, the SPOS 104 includes a control circuit 112and a switch mode rectifier circuit 114 as best shown in FIG. 5. Thecontrol circuit 112 includes a current sensor 116 coupled to the DEC 102and a voltage sensor 118 also coupled to the DEC 102. A second set ofvoltage and current sensors 116′, 118′ measure values from the load 106.The current sensors 116, 116′ could be an ammeter or a multi-meter. Thevoltage sensors 118, 118′ may be a voltmeter or a multi-meter.Alternatively, a pair of multi-meters could be used to measure both thevoltage and the current from the DEC 102 and the voltage and the currentfrom the switch mode rectifier circuit 114. The current sensors 116,116′ and the voltage sensors 118, 118′ are coupled to a controller 120.The controller 120 uses the current and voltage measurements from thesensors 116, 116′, 118, 118′ to drive the switch mode rectifier circuit114.

The switch mode rectifier circuit 114 includes a gate drive circuit 122which is coupled to the controller 120. The gate drive circuit 122generates the pulses for a power semiconductor switch 124 within theswitch mode rectifier circuit 114. The DEC 102 supplies the currentI_(S) to the power semiconductor switch 124 which may comprise powermetal-oxide semiconductor field effect transistor (MOSFET). It is to beunderstood that other types of switching devices 124 can be used withinthe scope of the invention, such as an insulated gate bipolar transistor(IGBT), bipolar transistor or power field effect transistor.

The power semiconductor switch 124 is coupled to a power diode 126 andan inductor 128. The inductor 128 is used to store excess energy duringthe on cycle of the power semiconductor switch 124. In the example ofFIG. 5, an inductance of approximately 10 milli-Henries was used, butthe inductance can vary depending upon system requirements. The powerdiode 126 is coupled in series to an output filter 130. The outputfilter 130 reduces ripple current and smoothes the DC output, (V_(out)).In the example of FIG. 5 the output filter 130 includes a resistor 127with a resistance of approximately 20 ohms and a capacitor 128 with acapacitance of approximately 470 mirco-Farads. The output filter 130 iscoupled in parallel to the energy storage device 106 of FIG. 4.

The storage device 106 is coupled to the SPOS 104 to provide some loadbalancing and to meet the load power demand by providing an energyreserve. In the example of FIG. 4, the storage device 106 shown is anultra-capacitor, however any other mechanism for storing energy such anelectrochemical battery could be employed. The storage device 106 isalso coupled to a DC/DC converter 108 for providing load regulation.

The load regulator 108 regulates the current flowing to the load 110.Further load leveling can also be achieved by incorporating theappropriate battery or capacitance across the load 110 if necessary.

The load 110 presents a complex impedance to the SPOS 104 (which can bewritten as Z_(L)=R_(L)+X_(L), where, R_(L) is the resistive or realpart, and X_(L) is the inductive/capacitive part). The load 110 couldalso be one of a fixed resistance, capacitance or inductance, Z_(L). Thesecond current and voltage sensors 116′, 118′ measure the current to theload I_(L) and the voltage across the load V_(L). The second sensors116′, 118′ transmit the current and voltage measurements to thecontroller 120.

The optimization system 100 functions by using the switch mode rectifiercircuit 114 to perform an impedance transformation based on input fromthe controller 120. In general terms, the controller 120 sends apulse-width modulated (PWM) signal based on an optimized value of thesource current Is from the DEC 102 to the gate drive circuit 122. Thegate drive circuit 122 sends a signal to the power semiconductor switch124, which then switches on and off at a rate determined by thecontroller 120. High efficiency power transfer is achieved by modulatingthe power semiconductor switch 124, which is turned on and off atfrequencies in the 10 kiloHertz range. The PWM signal created by thecontroller 120 has a duty cycle, d which is calculated based upon thevoltage and current measured by the sensors 116, 116′, 118, 118′. Thisresults in the power diode 126 going into conduction and non-conductionin a complementary manner.

Assuming that the current I_(S) is relatively constant over a PWM cycle,then the local average value of the voltage,

V₁

, is given by,

V₁

=(1−d)·V_(LOAD) and the local average of the output current,

I_(L)

, is given by,

I_(LOAD)

=(1−d)·I_(S). By controlling the duty cycle ratio, d, one can vary thelocal average voltage,

V₁

, to any value below V_(L). Thus, the switch mode rectifier circuit 114optimizes the current I_(S) from the DEC 102.

An exemplary routine for the controller 120 is shown in FIG. 6. Thecontroller 120 begins the optimization in step 200. Next, in step 202,the controller 120 measures the voltage and the current of the DEC 102and the load 110 from the sensors 116, 118, 116′, 118′. In step 204, thecontroller 120 calculates the source power P_(S), the load power P_(L),the source efficiency η and the load power transfer β. For a typicalthermal electric generator, the source efficiency η is given by$\eta_{T} = \frac{I_{S}{{}_{}^{}{}_{}^{}}}{{K\quad{\Delta T}} + {\alpha\quad T_{H}I_{S}} + {\frac{1}{2}I_{S}{{}_{}^{}{}_{}^{}}}}$where K is the thermal conductivity, ΔT is the thermal gradient acrossthe device and T_(H) is the hot side temperature. The load powertransfer β is defined as ${\beta = \frac{R_{L}}{R_{l}}},$where R_(I) is the combined impedance of DEC 102, regulator 108, SPOS104 and DEC 102 as seen from the load 110.

Next, at step 206 the controller 120 sets the PWM to yield a sourcecurrent I_(S) one preselected increment up or down in step 206. In step208, the controller 120 re-measures the voltage and the current of theDEC 102 and the load 110 from the sensors 116, 118, 116′, 118′. Thecontroller 120, in step 210, recalculates the source power P_(S), theload power P_(L), the source efficiency η and the load power transfer β.In step 212, the controller 120 determines if the product of the sourceefficiency η and the load power transfer β has changed. If the productof the source efficiency η and the load power transfer β has notchanged, then the controller 120 goes to step 214. In step 214, thecontroller 120 reverses the step direction of the source current I_(S)(up to down, or down to up) and sets the PWM to the gate drive circuit122 to yield a source current I_(S) one increment up or down from theprevious value. The controller 120 then loops to step 208.

If the ηβ product has changed, then the controller 120 goes to step 216.In step 216, if the product has increased and I_(S) was incremented up,then the controller 120 goes to decision block 218. In step 218, thecontroller 120 sets the PWM to gate drive to yield Is one increment downfrom its previous value. Then the controller 120 loops to step 208.

If at block 216 the product did not increase with I_(S) incremented up,then the controller 120 moves to step 220. In step 220, the controller120 sets the PWM to gate drive to yield I_(S) one increment up from itsprevious value. The controller 120 then loops to step 208, and thepolarity of the incrementation remains unchanged.

The optimization system 100 increases the efficiency of the DEC 102 byabout 50% for typical loads under continuous operation. The optimizationconfiguration 100 for the DEC 102 also enables both source and loadregulation, resulting in optimum power delivered to the load 110.Furthermore, the design of the switch mode rectifier circuit 114 isversatile enough to achieve superior performance especially for highpower and hybrid vehicle applications, however, other designs arepossible such as a conventional buck-boost or Cuk non-isolated DC/DCconverter.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. In a system for supplying electrical energy to a load from a directelectrical energy converter, an arrangement for optimizing converterpower generation efficiency comprising: an impedance transformationcircuit coupled between the energy converter and load for regulatingcurrent delivered by the energy converter so as to maximize powerdelivered to the load.
 2. The arrangement of claim 1 wherein theimpedance transformation circuit comprises: a power switch operative ina first state to conduct current from the energy converter to the loadand operative in a second state to inhibit energy converter current fromreaching the load; and a controller operative to place the power switchin its first and second states in accordance with a desired duty cycle.3. The arrangement of claim 2 wherein the controller further comprises:an energy converter output current sensor; an energy converter outputvoltage sensor; and a stored program processor unit coupled for receiptof signals from the energy converter output current and output voltagesensors and operative to regulate the desired duty cycle in accordancewith received sensor signals.
 4. The arrangement of claim 3 wherein thecontroller further comprises: a load current sensor coupled to theprocessor unit; and a load voltage sensor coupled to the processor unit;whereby the sensor signals utilized by the processor unit to regulatethe desired duty cycle further include signals from the load current andvoltage sensors.
 5. The arrangement of claim 1 further comprising: aload voltage regulator coupled between an output of the impedancetransformation circuit and the load.
 6. The arrangement of claim 5wherein the load voltage regulator comprises a DC/DC converter.
 7. Thearrangement of claim 5 further comprising: a load balancing energystorage device coupled across the output of the impedance transformationcircuit and an input of the load voltage regulator.
 8. The arrangementof claim 7 wherein the load balancing energy storage device comprises abattery.
 9. The arrangement of claim 7 wherein the load balancing energystorage device comprises an ultra-capacitor.
 10. The arrangement ofclaim 1 wherein the energy converter is selected from the groupconsisting of: fuel cell, thermoelectric or thermoionic device,electrochemical battery, solar cell or photovoltaic converter,thermophotovoltaic system, plasma power generator, ferroelectric device,piezoelectric device, electrohydrodynamic generator.
 11. A method ofoptimizing power generation efficiency of a direct electrical energyconverter applying electrical current to a load, the method comprising:monitoring output current and output voltage of the direct electricalenergy converter; monitoring current through and voltage across theload; placing an impedance transformation circuit between the directelectrical energy converter and the load; and adjusting impedance of theimpedance transformation circuit as a function of monitored energyconverter current and voltage and load current and voltage so as tomaximize power delivered to the load.
 12. The method of claim 11 whereinimpedance of the impedance transformation circuit is adjusted byaltering a duty cycle of a power switch operative in a first state todeliver the energy converter current to the load and operating in asecond state to inhibit energy converter current from reaching the load.13. The method of claim 12 wherein altering the duty cycle comprises:changing a value of energy converter current delivered to the load by apredetermined amount, the polarity of the predetermined amount dependingon whether a product of energy efficiency of the energy converter andpower transfer efficiency from the energy converter to the load haschanged.
 14. The method of claim 13 wherein the polarity of thepredetermined amount is reversed whenever the product has not changed.15. The method of claim 13 wherein the polarity of the predeterminedamount is reversed whenever the product changes positively and a presentpolarity of the predetermined amount is positive.
 16. The method ofclaim 13 wherein the polarity of the predetermined amount is reversedwhenever the product changes negatively and a present polarity of thepredetermined amount is negative.
 17. The method of claim 13 wherein thepolarity of the predetermined amount is unchanged whenever the productchanges in a direction opposite to the present polarity of thepredetermined amount.